In history class, the girl to boy ratio is $10$ to $10$. If there are a total of $40$ students, how many girls are there?
Explanation: A ratio of $10$ girls to $10$ boys means that a set of $20$ students will have $10$ girls and $10$ boys. A class of $40$ students has $2$ sets of $20$ students. Because we know that there are $10$ girls in each set of $20$ students, the class must have $2$ groups of $10$ girls each. There is a total of $20$ girls in history class.